Heathrow said it will be working with Border Force to strengthen contingency plans to minimise impact to customers’ journeys. 

"We want to reassure passengers that most journeys will be unaffected," a spokesperson added. "Those arriving on strike days, particularly those not eligible to use eGates, should be prepared to experience longer wait times at immigration."

Meanwhile, Gatwick said despite the walkouts, flights are expected to arrive and depart to and from the airport as normal, with other government staff assigned to operate passport checks at the border.

"However, passport checks for arriving passengers will take longer than usual, so passengers are advised to use e-gates where possible," a spokesperson said.

"Gatwick will also have extra staff available to help arriving passengers, who are advised to bring extra food and water in case of significant queues."

Manchester airport warned customers queues "may be longer than usual" due to the strike action, but is not anticipating any cancellations and passengers have been told to not change their travel plans unless advised to.